I think I would put in a bunch of -100F antifreeze rather than salt. I don't think salt will do anything to prevent the solids from freezing. (dunno if anything will) but also if you completely saturated it w/salt it would still freeze @ -21.1c Vs -73c for antifreeze, and remember that you'll not be able to saturate the tank w/Salt, and you'll also be diluting antifreeze and raising those temps considerably.

Because urine is so corrosive, any metal black water tank that's more than a couple of years old is living on borrowed time...so leaving the tank full for the winter, even without salt, is not a good plan. Adding salt certainly won't extend the tank life...and depending how cold it can get and how much is in the tank, it may take a LOT more than just a couple of pounds to provide any protection from freezing.

So you really do need empty the tank AND thoroughly flush it out before the boat sits...no easy task, judging from the size of your boat. But if you can get a portable pump and water hose to it, or a barrel to catch discharge from a macerator pump, I urge you to do it.